Python的string模块中的Template类字符串模板用法

string.Template()
string.Template()内添加替换的字符, 使用"$"符号, 或 在字符串内, 使用"${}"; 调用时使用string.substitute(dict)函数.
可以通过继承"string.Template", 覆盖变量delimiter(定界符)和idpattern(替换格式), 定制不同形式的模板.

代码:

# -*- coding: utf-8 -*- 

import string 
 
template_text = ''''' 
  Delimiter : %% 
  Replaced : %with_underscore 
  Ingored : %notunderscored 
''' 
 
d = {'with_underscore' : 'replaced', 
   'notunderscored' : 'not replaced'} 
 
class MyTemplate(string.Template): 
  delimiter = '%' 
  idpattern = '[a-z]+_[a-z]+' 
   
t = MyTemplate(template_text) 
print('Modified ID pattern: ') 
print(t.safe_substitute(d))

输出:

Modified ID pattern:  
 
  Delimiter : % 
  Replaced : replaced 
  Ingored : %notunderscored

注意: 定界符(delimiter)为"%", 替换模式(idpattern)必须包含下划线, 所以第2个没有进行替换.

正则替换

string.Template的pattern是一个正则表达式, 可以通过覆盖pattern属性, 定义新的正则表达式.
如: 使用新的定界符"{{", 把{{var}}作为变量语法.

代码:

import string 
 
t = string.Template('$var') 
print(t.pattern.pattern) 
 
class MyTemplate(string.Template): 
  delimiter = '{{' 
  pattern = r''''' 
  \{\{(?: 
   (?P<escaped>\{\{) |  # Escape sequence of two delimiters 
   (?P<named>[_a-z][_a-z0-9]*)\}\}   |  # delimiter and a Python identifier 
   {(?P<braced>[_a-z][_a-z0-9]*)}\}\}  |  # delimiter and a braced identifier 
   (?P<invalid>)       # Other ill-formed delimiter exprs 
  ) 
  ''' 
   
t2 = MyTemplate(''''' 
{{{{ 
{{var}} 
''') 
 
print('MATCHES: ', t2.pattern.findall(t2.template)) 
print('SUBSTITUTED: ', t2.safe_substitute(var='replacement'))

输出:

\$(?: 
   (?P<escaped>\$) |  # Escape sequence of two delimiters 
   (?P<named>[_a-z][_a-z0-9]*)   |  # delimiter and a Python identifier 
   {(?P<braced>[_a-z][_a-z0-9]*)}  |  # delimiter and a braced identifier 
   (?P<invalid>)       # Other ill-formed delimiter exprs 
  ) 
   
MATCHES: [('{{', '', '', ''), ('', 'var', '', '')] 
SUBSTITUTED:  
{{ 
replacement

字符串模板的安全替换(safe_substitute)
字符串模板(sting.Template), 替换时, 使用substitute(), 未能提供模板所需的全部参数值时, 会发生异常.
如果使用safe_substitute(), 即安全替换, 则会替换存在的字典值, 保留未存在的替换符号.

代码:

import string 
 
values = {'var' : 'foo'} 
 
t = string.Template('''''$var is here but $ missing is not provided! ''') 
 
 
try: 
  print 'substitute() : ', t.substitute(values) 
except ValueError as err: 
  print 'Error:', str(err) 
   
print 'safe_substitude() : ', t.safe_substitute(values)

输出:

substitute() : Error: Invalid placeholder in string: line 1, col 18 
safe_substitude() : foo is here but $ missing is not provided!